1. Field of the Invention
This invention pertains to gimbals and, more particularly, to a gimbal having structural axes of rotation which intersect nonorthogonally.
2. Description of the Prior Art
The use of a gimbal to maintain a steady line of sight for an instrument mounted on a moving vehicle is well known. Due to the large distance typically separating a target located in the instrument's line of sight and the instrument, relative translation between the target and the instrument is of little consequence. However, the line of sight is quite sensitive to relative angular motion between the instrument and the target, and the gimbal has been used to mount the instrument on the moving vehicle in an effort to solve this problem. The gimbals of the prior art have used three orthogonal axes of rotation, one each coincident with the pitch, roll, and yaw axes, respectively, of the vehicle. This has given rise to a problem, however, because the structure necessary to support one of the axes typically blocks the line of sight of the instrument, commonly termed "vignetting."
Several designs have been used in attempting to overcome the vignetting problem. One employs a cantilevered Y-shaped yoke which holds the instrument in the fork of the yoke. The stem of the yoke is colinear with the instrument's line of sight, and is cantilevered from a pair of bearings to allow rotation of the stem. The apparatus thus allows an unobstructed line of sight for the instrument while providing the instrument with a rotational degree of freedom about the line of sight.
There are two typical configurations which incorporate this design. The first holds the stem between a pair of annular bearings spaced some distance apart from each other. This configuration minimizes frictional torque because the bearing diameters are small. However, bending stiffness is compromised because the shaft diameter is necessarily small in order to fit within the bearings. The reduced bending stiffness allows an increase in the amplitude of the bending of the shaft and yoke at a relatively low frequency, resulting in an undesirable angular motion of the line of sight.
The other configuration which uses a Y-shaped yoke passes the stem of the yoke through a pair of annular bearings located adjacent to each other. This alternative employs bearings having a large diameter and thus allows a large diameter shaft to be used, thereby significantly reducing the low frequency bending experienced in the first configuration. However, a higher frictional torque is generated, primarily from the increased radius of the bearings. The increased frictional torque proportionally decreases the servo's ability to stabilize the line of sight of the apparatus mounted on the gimbal. This results in a larger angular error than would otherwise be the case for a gimbal with a lower frictional torque.
Another attempt at solving this problem uses two spoked wheels to rotatively attach one end of the suspended instrument to the gimbal. One spoked wheel is fixedly situated in the end of the instrument from which the line of sight emanates, while the second wheel is spaced apart therefrom and is fixedly attached along its circumference to the gimbal (typically by being attached to the inner diameter of a cylinder which is in turn attached to the gimbal). Each wheel has an annular bearing at its center and a rotatable shaft passes through both of them. The other end of the instrument is rotatively attached to the gimbal by a second shaft which is attached to the instrument and extends therefrom to a bearing mounted on the gimbal structure. The instrument is thus supported between the two shafts and rotatively attached to the remainder of the gimbal structure. The two shafts lie upon the line of sight of the instrument, and thereby provide the instrument with the freedom to rotate about its line of sight.
The frictional torque for such a configuration can be made acceptably low and the structural stiffness can be made high enough to satisfy the criteria for most applications. The spoked wheels avoid total vignetting of the line of sight. However, the partial obstruction of the line of sight is nonetheless unacceptable for many applications, for example, a tracker with a defocused image or an infrared optical system. It should be noted that translucent glass discs may be substituted for the spoked wheels to further reduce the partial vignetting occasioned by the spokes of the spaced wheels. However, this variation of the spaced wheel concept may introduce undesirable optical effects.
Another approach is to use a spherical hydrostatic gas bearing having a large cylindrical passageway through the center of the suspended sphere to contain the instrument and leave its line of sight unobstructed. The sphere is supported on a layer of gas flowing from orifices in two opposing concave cups which are very closely spaced from the sphere. This configuration provides the lowest frictional torque and the highest structural stiffness of any design of the prior art.
There are, however, several drawbacks attendant to the use of a spherical hydrostatic gas bearing. The apparatus is very expensive because the sphere and cups must be fabricated to an extremely high degree of accuracy. A reservoir and valve or pump are necessary to supply a continuous flow of gas at the required pressure. As the cups continuously emit gas into the immediate environment, this concept cannot be used on many extraterrestrial missions because the gas would interfere with experiments. In addition, the gas bearing is very sensitive to the presence of foreign matter, for example, dust or oil, in between a cup and the sphere. More particularly, insertion of foreign matter in the space separating a cup and the sphere couples the sphere to the cup, which is in turn attached to the vehicle, and thereby allows vibration to be transmitted from the vehicle to the sphere.
The instrument has also been held in the center of a large ring bearing in an attempt to overcome the problems of vignetting. In this configuration, the inner diameter of the bearing holds the instrument while the outer diameter is attached to the remaining structure of the gimbal. The instrument is situated in the bearing so that its line of sight either coincides with or is parallel to the bearing's axis of symmetry. The problem inherent in this design is that the bearing typically has a relatively large radius and, concomitantly, a large frictional torque which, as previously explained, increases the angular error between the line of sight and the target.